Applicants claim, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Feb. 4, 1999 of a German patent application, copy attached, Serial Number 199 04 470.8, filed on the aforementioned date, the entire contents of which is incorporated herein by reference. Applicants also claim, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Dec. 1, 1999 of a German patent application, copy attached, Serial Number 199 57 777.3, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical position measuring system which is suitable for the precise determination of the relative position of two objects which can be moved with respect to each other.
2. Discussion of Related Art
Incremental position measuring systems are known, wherein the scanning graduations on the scale and the scanning sides, i.e., the measuring graduation and one or several scanning graduations, have different graduation periods. If these graduations are illuminated by a light source, a periodic fringe pattern results in one detector plane, which can be detected by means of a suitable detection arrangement. In this case, the periodic fringe pattern is the result of interactions of the beam emitted by the light source and the various graduations in the beam path. This fringe pattern will be called a vernier fringe pattern in what follows, wherein the periodicity of this fringe pattern is defined by the vernier period.
In connection with the manner of generating the vernier fringe pattern, these, for one, can be so-called imaging position measuring systems with relatively coarse graduation periods on the measuring graduation and the scanning side in this case. The resultant vernier fringe pattern is here essentially generated by the cast shadows. As a rule, these systems include a measuring graduation, as well as a scanning graduation; in this connection reference is made to DE 195 27 287A or DE 1 796 368 A1, for example. Scanning of the resultant vernier fringe pattern with a relatively large vernier period is respectively performed with the aid of suitably arranged quadrant detectors. Moreover, reference is made to DE 26 53 545 in this connection.
Furthermore, the resultant vernier fringe pattern, in principle, can also be generated by means of an interferential position measuring system, wherein the graduation structures on the graduation and the scanning side have very small graduation periods. With such measuring systems, the scanned vernier fringe pattern in the detector plane consists of partial beams, which are diffracted at the graduations used and come to interference. Reference is made in this connection to DE 27 14 324, for example.
It is now intended to disclose imagining position measuring systems, as well as interferential position measuring systems, for the generation of such vernier fringe patterns, which meet defined requirements.
It should be assured in principle that, in the case of a relative movement, sufficiently well modulated scanning signals result from scanning the vernier fringe pattern. Possible contamination of a portion of the graduation via such sources as dirt, soil, oil or coolants, should not have a negative effect on the scanning signals to the extent possible. Moreover, a certain flexibility regarding the position of the detector plane is required, since, for reasons of existing structural conditions, sometimes the detector plane cannot always be arranged immediately behind the last graduation of the position measuring system which was passed. The latter is of particular importance in view of compactly constructed scanning units. It should furthermore be noted that with smaller periods of the vernier fringe pattern in imaging systems, the distance between the last graduation which was passed and the detector plane should be as small as possible. The reasons for this are the higher orders of diffraction which lead to a fringe image with only low contrasts. However, in actual use such a short distance can hardly be realized; it would be possible that bonding wires of the respective detector elements, which project past the detector elements, might be damaged in this case, etc.
In the case of interferential systems, a spatial separation of the exiting orders of diffraction by means of lenses is often required. However, even in the case of different graduation periods of the graduations used, no vernier fringes appear in the focal planes of the lenses of such position measuring systems.
An optical position measuring system for determining the relative position of a first object which can be moved with respect to a second object to along a measuring direction which meets these requirements includes at least one periodic measuring graduation, which is connected to the first object and a scanning unit which is connected with the second object. The scanning unit includes a light source emitting beams of light, at least one scanning graduation and a detector arrangement in a detector plane. The detector arrangement includes several radiation sensitive detector elements for scanning a periodic fringe pattern resulting from the interaction of the beams of light emitted by the light source with the at least one periodic measuring graduation and the at least one scanning graduation, wherein the detector plane is arranged spaced at a distance Zn from the last graduation passed and the distance Zn is calculated from the following equation:
1/Zn+1/ZQ=1/((n+xcex7)*dVTO)),
wherein:
ZQ: is the distance of the last graduation passed from a real and virtual source point of the periodic fringe pattern,
n=0, 1, 2, 3, . . . ,
xcex7: is the phase shift in fractions of 360xc2x0 of the periodic fringe pattern exiting at the last graduation passed into different directions,
dVTO=(TPeff*xcex9vernier)/xcex,
xe2x80x83wherein
TPeff: is an effective graduation period of the scanning device, which correctly describes the directions of the orders of diffraction exiting at the graduations last passed, which have sufficient intensity,
xcex: wavelength of said light source,
xcex9vernier: period of said partial vernier fringe at the location of the last graduation passed.
The steps in accordance with the invention, in particular on the part of the detector arrangement, now assure a high-resolution generation of the displacement-dependent scanning signals, which is insensitive to contamination, or respectively insensitive to malfunctions. This is assured by means of the appropriately designed detector arrangement, since possible contamination of the graduation has equal effects on the different phase-shifted signal portions to a large degree.
Furthermore, knowing the optimal position of the detector plane results in a definite flexibility in view of the most diverse structural situations. For example, it is no longer absolutely necessary to place the detector arrangement directly following the last graduation which was respectively passed through; instead, by means of the present invention that there are further options for arranging the detector elements, which also provide sufficiently well modulated scanning signals.
The result of this in the end is the possibility of realizing a very compactly constructed scanning unit with a simultaneously great signal contrast, or respectively degree of modulation.
In the same way, knowing the optimal position of the detector plane, it is now possible to disclose an interferential position measuring system, which provides a vernier fringe pattern on the scanning side. It should furthermore be noted as an advantage of the interferential position measuring systems in accordance with the invention, in which no separation of the different orders of diffraction is performed, that all signal portions are equally affected, if the diffraction properties should change over the length of the graduation.
It is moreover possible in this way to generate signals with respective phase shifts of 90xc2x0 in interferential systems, which can be further processed in standard electronic evaluation devices. Furthermore, by means of the very precise generation of differential mode signals it is possible to eliminate the second harmonic, which otherwise causes errors in the subsequent signal interpolation.
The inventive concepts can of course also be applied to rotary, as well as linear position measuring systems. It is also possible to design position measuring systems in accordance with the invention, which operate in incident or transmitted light.
Further advantages as well as details of the optical position measuring systems in accordance with the invention ensue from the following description of several exemplary embodiments by means of the attached drawings.